Diesel oxidation catalyst (DOC) systems, DPF systems, NOx occlusion reduction catalyst (lean NOx trap (LNT) or NOx storage reduction (NSR)) systems, urea selective catalytic reduction (SCR) systems, and so on are in practical use for exhaust gas aftertreatment devices of diesel engines.
Out of the above-mentioned systems, the DOC systems and the DPF systems are effective systems for reducing PM. Although the DOC, which is provided at an upstream position in the exhaust passage, is not capable of oxidizing solid soot, the DOC oxidizes a large portion of soluble organic fraction (SOF), which accounts for 30 to 70% of the total PM, and also removes hydro carbon (HC) and CO at the same time. The DPF, which is provided at a downstream position, is formed of porous ceramics or the like having a fine pore size and captures a large portion of the PM contained in the exhaust gas.
When an amount of PM captured and accumulated in the DPF reaches a predetermined amount, regeneration is forcibly applied to the DPF in order to remove the accumulated PM. The regeneration of the DPF is carried out by multiple-injection, post injection, exhaust pipe injection or the like such that the fuel is oxidized and burned in the DOC to raise the exhaust gas temperature, and the PM accumulated in the DPF is burned and removed.
The exhaust pipe injection has advantages over fuel injection into cylinders during an expansion stroke of combustion (so-called post injection). For example, the fuel does not dilute the oil and the entire amount of injected fuel can be used for temperature elevation so that the fuel efficiency deterioration in connection with the temperature elevation is reduced.